Flow charts are a diagrammatic representation of a set of instructions which must be followed. Flow charts are made up of different boxes, which each have different functions.

The flow chart above says think of a number, add 5 and multiply by 2. If the number is negative, make it positive.
Flow charts are usually equivalent to some function. In the example above, the flow chart is equivalent to the function f(x) = | 2(x + 5) | .
(the vertical lines around 2(x + 5) means take the magnitude of the answer).